Vari-focal lens system

ABSTRACT

A vari-focal lens system comprising, in the order from the object side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power and a fourth lens unit; adapted to perform variation of focal length by moving the second lens unit and the third lens unit along the optical axis; and adapted to perform focusing by moving the second lens unit toward the object side.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The present invention relates to a vari-focal lens system which is to befocused by the rear focusing method or the inner focusing method.

b) Description of the Prior Art

The conventional zoom lens system is focused by moving the front lensunit along the optical axis. This is because the moving distancerequired for focusing varies along with variation of the focal length ofthe zoom lens system as a whole when the zoom lens system is focused bythe inner focusing method or the rear focusing method, i.e., by moving alens unit which is arranged on the image side of the lens unit havingthe vari-focal function in the zoom lens system. Accordingly, the zoomlens system which adopts the rear or inner focusing method does notallow graduation of the distances to objects to be brought into focus incorrespondence to the moving distances of the focusing lens unit.Further, the conventional zoom lens system is defocused by changingfocal length thereof after the zoom lens system is focused on an objectlocated at a certain distance.

However, most of the still cameras and video cameras are equippednowadays with autofocusing mechanisms. Therefore, the above-mentionedproblem posed by adopting the rear focusing method or the inner focusingmethod can be corrected by using, as a focus detecting system, theso-called TTL type focusing system or the autofocusing system which isequipped with a focal length encoder for zoom lens systems, a ROM forstoring lens system operating data, a calculating mechanism and acontrol mechanism.

Furthermore, the conventional zoom lens systems comprise a front lensunit which is generally large and heavy. Therefore, this lens unit ismoved slowly for focusing, thereby slowing down focusing speed. In orderto correct this problem, it is necessary to strengthen the power of thedriving system though it results in a disadvantage of enhancing powerconsumption.

Accordingly, a merit is obtained for the autofocusing cameras byadopting the rear focusing method or the inner focusing method whichperforms focusing by moving a rear or intermediate lens unit which issmall and light in weight. For this reason, there have been proposednumerous zoom lens systems which adopt the rear focusing method or havethe compositions suited for the rear focusing method. From themechanical viewpoint, however, it is desirable for a zoom lens system tohave a composition as simple as possible.

As conventional examples of zoom lens systems each of which uses onlytwo movable lens units for both the variation of focal length andfocusing, there are known the lens systems disclosed by Japanese PatentKokai Publication No. Sho 62-178917 and Japanese Patent KokaiPublication No. Sho 63-29718. Each of these conventional examplesconsists, in the order from the object side, of a first lens unit havinga positive refractive power, a second lens unit which has a negativerefractive power and a vari-focal function, a third lens unit having apositive refractive power, and a fourth lens unit which has a positiverefractive power, and is moved for focusing and for correcting thevariation of image point caused due to the variation of focal length.Since the fourth lens unit of this conventional example has an imagingmagnification lower than 1x, the zoom lens system is adapted so as tomove the fourth lens unit toward the object side when the zoom lenssystem is to be focused on an object located at a short distance.

Moreover, a certain zoom lens system comprises a wide airspace betweenthe third lens unit and the fourth lens unit. However, it is desired forsuch a zoom lens system to reduce the airspace as much as possible so asto meet the recent demand for compact design of the zoom lens system.

In addition, since the paraxial ray is high on the fourth lens unit, azoom lens system which has a small F number or a large aperture iseasily influenced in optical performance by eccentricity of lens elementand can hardly be manufactured in practice. Especially, considering thefact that the image pickup devices which recently have small sizesimpose strict manufacturing precision on lens elements, it isundesirable to select the rear focusing method or moving the fourth lensunit for focusing. The rear focusing method is still undesirable for thesame reason even when the role of the fourth lens unit is allotted tothe third lens unit.

SUMMARY OF THE INVENTION

A primary object of the present invention is to provide a vari-focallens system which is to be focused by the rear focusing method or theinner focusing method, and is designed so as to reserve an airspace formoving lens units and have a composition allowing little influence onoptical performance due to eccentricity of lens elements.

The vari-focal lens system according to the present invention comprises,in the order from the object side, a first lens unit having a positiverefractive power, a second lens unit having a negative refractive power,a third lens unit having a positive refractive power and a fourth lensunit, and is adapted so as to perform zooming by moving the second lensunit and the third lens unit, said second lens unit satisfying thecondition (1) mentioned below and moved toward the object side forfocusing the vari-focal lens system.

    |β.sub.IIT |<0.95                   (1)

wherein the reference symbol β_(IIT) represents the magnification of thesecond lens unit when the vari-focal lens system is focused on an objectlocated at an infinite distance.

The vari-focal lens system according to the present invention comprisesthe second lens unit and the third lens unit which are movable forvariation of focal length as described above, and adopts the rearfocusing method which moves the second lens unit, one of the lens unitsmovable for focusing. Owing to this design, the airspace reserved formoving the movable lens units for variation of focal length can beutilized also for moving the second lens unit for focusing.

When the vari-focal lens system is designed so as to perform focusing bymoving the second lens unit as described above, the moving distance ofthe second lens unit for focusing at the tele position is far longerthan that at the wide position, or equal to the product of the movingdistance at the wide position multiplied by the square of the vari-focalratio.

However, the airspace reserved between the first lens unit and thesecond lens unit is widened as the vari-focal lens system as a wholeacquires a longer focal length. Accordingly, no surplus airspace isnecessary for focusing when the second lens unit is moved toward theobject side (toward the first lens unit) for focusing the vari-focallens system from an object located at an infinite distance onto anotherobject located at a short distance. In order to focus the vari-focallens system onto the object located at the short distance by moving thesecond lens unit toward the object side, it is sufficient to design thefocusing lens unit so as to have a magnification lower than 1x.

Since the imaging magnification of the second lens unit is the highestat the tele position, it is sufficient for the purpose mentioned aboveto define the imaging magnification of the second lens unit to be setwhen the vari-focal lens system is focused on an object located at aninfinite distance at the tele position thereof. For the reason describedabove, it is desirable to select the imaging magnification β_(IIT) ofthe second lens unit so as to satisfy the condition (1).

If the imaging magnification β_(IIT) has an absolute value larger than0.95 and close to 1, the moving direction of the second lens unit(focusing lens unit) will be indefinite, and when the absolute value isfar larger than 1, the second lens unit will be moved in the reversedirection and will interfere mechanically with the third lens unit.Further, inconvenience will be produced since the moving direction ofthe second lens unit will be reversed between the wide position and thetele position, and indefinite at certain intermediate focal lengths.

When the above-mentioned condition (1) is satisfied, the second lensunit can be moved for focusing toward the object side where a sufficientairspace is reserved. In this case, however, aberrations may be varied,especially remarkable in the vicinity of the tele position.

When the second lens unit has a focal length which is extremely short ascompared with that of the vari-focal lens system as a whole, sphericalaberration and coma will undesirably be varied remarkably. It isdesirable for the second lens unit to have a focal length satisfying thefollowing condition (2):

    0.15<|f.sub.II |/f.sub.T <0.5            (2)

wherein the reference symbol f_(II) represents the focal length of thesecond lens unit and the reference symbol f_(T) designates the focallength of the vari-focal lens system as a whole at the tele position.

If the lower limit of the condition (2) is exceeded, sphericalaberration and coma will tend to be varied remarkably by focusing. Ifthe upper limit of the condition (2) is exceeded, the imagingmagnification of the seocnd lens unit can hardly be kept lower than 1xover the entire vari-focal range, thereby producing a tendency toprolong and enlarge the total length of the vari-focal lens system andthe diameter of the first lens unit respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 through FIG. 4 show sectional views illustrating compositions ofEmbodiments 1 through 4 of the present invention;

FIG. 5, FIG. 6 and FIG. 7 show graphs illustrating aberrationcharacteristics at the wide position, intermediate focal length and teleposition respectively of Embodiment 1 of the present invention when itis focused on an object located at an infinite distance;

FIG. 8, FIG. 9 and FIG. 10 show graphs illustrating aberrationcharacteristics at the wide position, intermediate focal length and teleposition respectively of Embodiment 1 when it is focused on an objectlocated at a distance of 1.0 m;

FIG. 11, FIG. 12 and FIG. 13 show curves illustrating aberrationcharacteristics at the wide position, intermediate focal length and teleposition respectively of Embodiment 2 of the present invention when itis focused on an object located at an infinite distance;

FIG. 14, FIG. 15 and FIG. 16 show curves illustrating aberrationcharacteristics at the wide position, intermediate focal length and teleposition respectively of Embodiment 2 when it is focused on an objectlocated at a distance of 1.0 m;

FIG. 17, FIG. 18 and FIG. 19 show graphs visualizing aberrationcharacteristics at the wide position, intermediate focal length and teleposition respectively of Embodiment 3 of the present invention when itis focused on an object located at infinite distance;

FIG. 20, FIG. 21 and FIG. 22 show curves visualizing aberrationcharacteristics at the wide position, intermediate focal length and teleposition respectively of Embodiment 3 when it is focused on an objectlocated at a distance of 1.0 m;

FIG. 23, FIG. 24 and FIG. 25 show graphs visualizing aberrationcharacteristics at the wide position, intermediate focal length and teleposition respectively of Embodiment 4 of the present invention when itis focused on an object located at an infinite distance; and

FIG. 26, FIG. 27 and FIG. 28 show curves illustrating aberrationcharacteristics at the wide position, intermediate focal length and teleposition respectively of Embodiment 4 of the present invention when itis focused on an object located at a distance of 1.0 m.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, numerical data of the Embodiments 1 through 4 of the presentinvention will be described below.

Embodiment 1

    ______________________________________                                        f = 9.263˜26.218, F/2.7                                                 2ω = 48.8°˜18.2°                                    ______________________________________                                        r.sub.1 = 20.6250                                                                       d.sub.1 = 1.5000                                                                            n.sub.1 = 1.84666                                                                        ν.sub.1 = 23.78                         r.sub.2 = 15.8200                                                                       d.sub.2 = 0.4300                                                    r.sub.3 = 17.4930                                                                       d.sub.3 = 4.8000                                                                            n.sub.2 = 1.69680                                                                        ν.sub.2 = 55.52                         r.sub.4 = ∞                                                                       d.sub.4 = D.sub.1 (variable)                                        r.sub.5 = 119.7480                                                                      d.sub.5 = 0.9000                                                                            n.sub.3 = 1.69680                                                                        ν.sub.3 = 55.52                         r.sub.6 = 8.0570                                                                        d.sub.6 = 2.4000                                                    r.sub.7 = -21.0120                                                                      d.sub.7 = 0.9000                                                                            n.sub.4 = 1.69680                                                                        ν.sub.4 = 55.52                         r.sub.8 = 25.9270                                                                       d.sub.8 = D.sub.2 (variable)                                        r.sub.9 = 17.3500                                                                       d.sub.9 = 2.0000                                                                            n.sub.5 = 1.84666                                                                        ν.sub.5 = 23.78                         r.sub.10 = ∞                                                                      d.sub.10 = D.sub.3 (variable)                                       r.sub.11 = ∞ (stop)                                                               d.sub.11 = 1.5000                                                   r.sub.12 = 10.7720                                                                      d.sub.12 = 4.6000                                                                           n.sub.6 = 1.74950                                                                        ν.sub.6 = 35.27                         r.sub.13 = -16.5440                                                                     d.sub.13 = 0.2100                                                   r.sub.14 = -11.3480                                                                     d.sub.14 = 1.0000                                                                           n.sub.7 = 1.84666                                                                        ν.sub.7 = 23.78                         r.sub.15 = 11.3480                                                                      d.sub.15 = 2.6200                                                   r.sub.16 = 43.0900                                                                      d.sub.16 = 3.2000                                                                           n.sub.8 = 1.69680                                                                        ν.sub.8 = 55.52                         r.sub.17 = -12.1820                                                                     d.sub.17 = 0.1500                                                   r.sub.18 = 43.5920                                                                      d.sub.18 = 2.2000                                                                           n.sub.9 = 1.69680                                                                        ν.sub.9 = 55.52                         r.sub.19 = - 56.1520                                                                    d.sub.19 = 4.9500                                                   r.sub.20 = ∞                                                                      d.sub.20 = 7.9000                                                                           n.sub.10 = 1.51633                                                                       ν.sub.10 = 64.15                        r.sub.21 = ∞                                                                      d.sub.21 = 1.2000                                                                           n.sub.11 = 1.51633                                                                       ν.sub.11 = 64.15                        r.sub.22 = ∞                                                                      d.sub.22 = 5.1000                                                                           n.sub.12 = 1.54771                                                                       ν.sub.12 = 62.83                        r.sub.23 = ∞                                                                      d.sub.23 = 0.9000                                                   r.sub.24 = ∞                                                                      d.sub.24 = 0.7000                                                                           n.sub.13 = 1.51633                                                                       v.sub.13 = 64.15                           r.sub.25 = ∞                                                                      d.sub.25 = 0.3100                                                   r.sub.26 = ∞                                                                      d.sub.26 = 0.6000                                                                           n.sub.14 = 1.48749                                                                       ν.sub.14 = 70.20                        r.sub.27 = ∞                                                            ______________________________________                                        f       9.263         15.192  26.218                                          D.sub.1 0.600         7.250   13.190                                          D.sub.2 0.930         1.470   0.800                                           D.sub. 3                                                                              13.760        6.570   1.300                                           ______________________________________                                               D.sub.1W = 0.493, D.sub.1T = 12.564                                           |β.sub.IIT | = 0.617, |f.sub.II               |/f.sub.T = 0.248                                             ______________________________________                                    

Embodiment 2

    ______________________________________                                        f = 9.259˜26.147, F/2.7                                                 2ω = 48.8°˜18.2°                                    ______________________________________                                        r.sub.1 = 58.7060                                                                       d.sub.1 = 1.6000                                                                            n.sub.1 = 1.84666                                                                        ν.sub.1 = 23.78                         r.sub.2 = 32.7940                                                                       d.sub.2 = 0.0800                                                    r.sub.3 = 34.0170                                                                       d.sub.3 = 3.6000                                                                            n.sub.2 = 1.60311                                                                        ν.sub.2 = 60.70                         r.sub.4 = -296.6310                                                                     d.sub.4 = 0.1500                                                    r.sub.5 = 27.3950                                                                       d.sub.5 = 2.9000                                                                            n.sub.3 = 1.69680                                                                        ν.sub.3 = 55.52                         r.sub.6 = 126.2960                                                                      d.sub.6 = D.sub.1 (variable)                                        r.sub.7 = 94.2810                                                                       d.sub.7 = 0.9000                                                                            n.sub.4 = 1.83400                                                                        ν.sub.4 = 37.16                         r.sub.8 = 9.3580                                                                        d.sub.8 = 2.5000                                                    r.sub.9 = -13.1840                                                                      d.sub.9 = 0.9000                                                                            n.sub. 5 = 1.74320                                                                       ν.sub.5 = 49.31                         r.sub.10 = 13.1840                                                                      d.sub.10 = 2.2000                                                                           n.sub.6 = 1.84666                                                                        ν.sub.6 = 23.78                         r.sub.11 = -93.3350                                                                     d.sub.11 = D.sub.2 (variable)                                       r.sub.12 = 47.8790                                                                      d.sub.12 = 1.6000                                                                           n.sub.7 = 1.83400                                                                        ν.sub.7 = 37.16                         r.sub.13 = -98.2020                                                                     d.sub.13 = D.sub.3 (variable)                                       r.sub.14 = ∞ (stop)                                                               d.sub.14 = 1.6000                                                   r.sub.15 = 13.8640                                                                      d.sub.15 = 2.8000                                                                           n.sub.8 = 1.70154                                                                        ν.sub.8 = 41.21                         r.sub.16 = -68.4970                                                                     d.sub.16 = 0.4600                                                   r.sub.17 = -13.7800                                                                     d.sub.17 = 6.0000                                                                           n.sub.9 = 1.80518                                                                        ν.sub.9 = 25.43                         r.sub.18 = 20.9170                                                                      d.sub.18 = 0.3800                                                   r.sub.19 = 54.9350                                                                      d.sub. 19 = 2.8000                                                                          n.sub.10 = 1.69680                                                                       ν.sub.10 = 55.52                        r.sub.20 = -14.9350                                                                     d.sub.20 = 0.1500                                                   r.sub.21 = 31.2930                                                                      d.sub.21 = 2.5000                                                                           n.sub.11 = l.69680                                                                       ν.sub.11 = 55.52                        r.sub.22 = -31.2930                                                                     d.sub.22 = 13.1500                                                  r.sub.23 = ∞                                                                      d.sub.23 = 1.0000                                                                           n.sub.12 = 1.51633                                                                       ν.sub.12 = 64.15                        r.sub.24 = ∞                                                                      d.sub.24 = 4.8000                                                                           n.sub.13 = 1.54771                                                                       ν.sub.13 = 62.83                        r.sub.25 = ∞                                                                      d.sub.25 = 1.0000                                                   r.sub.26 = ∞                                                                      d.sub.26 = 0.7000                                                                           n.sub.14 = 1.51633                                                                       ν.sub.14 = 64.15                        r.sub.27 = ∞                                                                      d.sub.27 = 0.3100                                                   r.sub.28 = ∞                                                                      d.sub.28 = 0.6000                                                                           n.sub.15  = 1.48749                                                                      ν.sub.15 = 70.20                        r.sub.29 = ∞                                                            ______________________________________                                        f       9.259         15.633  26.147                                          D.sub.1 1.000         8.306   13.857                                          D.sub.2 0.640         1.960   0.600                                           D.sub.3 14.117        5.491   1.300                                           ______________________________________                                               D.sub.1W = 0.820, D.sub.1T = 12.274                                           |β.sub.IIT | = 0.845, |f.sub.II               |/.sub.fT = 0.308                                             ______________________________________                                    

Embodiment 3

    ______________________________________                                        f = 10.311˜26.211, F/2.7                                                2ω = 44.4°˜18.2°                                    ______________________________________                                        r.sub.1 = 23.6800                                                                       d.sub.1 = 1.5000                                                                            n.sub.1 = 1.84666                                                                        ν.sub.1 = 23.78                         r.sub.2 = 16.8630                                                                       d.sub.2 = 0.2800                                                    r.sub.3 = 18.1330                                                                       d.sub.3 = 4.6000                                                                            n.sub.2 = 1.72916                                                                        ν.sub.2 = 54.68                         r.sub.4 = ∞                                                                       d.sub.4 = D.sub.1 (variable)                                        r.sub.5 = 42.4050                                                                       d.sub.5 = 0.9000                                                                            n.sub.3 = 1.69680                                                                        ν.sub.3 = 56.49                         r.sub.6 = 9.1690                                                                        d.sub.6 = 2.5900                                                    r.sub.7 = -14.2020                                                                      d.sub.7 = 0.8000                                                                            n.sub.4 = 1.61700                                                                        ν.sub.4 = 62.79                         r.sub.8 = 66.6990                                                                       d.sub.8 = D.sub.2 (variable)                                        r.sub.9 = 27.1360                                                                       d.sub.9  = 1.7000                                                                           n.sub.5 = 1.84666                                                                        ν.sub.5 = 23.78                         r.sub.10 = ∞                                                                      d.sub.10 = D.sub.3 (variable)                                       r.sub.11 = ∞ (stop)                                                               d.sub.11 = 1.8000                                                   r.sub.12 = 30.3850                                                                      d.sub.12 = 2.1000                                                                           n.sub.6 = 1.78470                                                                        ν.sub.6 = 26.22                         r.sub.13 = -15.2330                                                                     d.sub.13 = 0.3500                                                   r.sub.14 = -8.6190                                                                      d.sub.14 = 4.5500                                                                           n.sub.7 = 1.80518                                                                        ν.sub.7 = 25.43                         r.sub.15 = 22.8320                                                                      d.sub.15 = 0.0700                                                   r.sub.16 = 27.0080                                                                      d.sub.16 = 3.4000                                                                           n.sub.8 = 1.61700                                                                        ν.sub.8 = 62.79                         r.sub.17 = -10.3220                                                                     d.sub.17 = 0.1500                                                   r.sub.18 = 56.5600                                                                      d.sub.18 = 2.1000                                                                           n.sub.9 = 1.60311                                                                        ν.sub.9 = 60.70                         r.sub.19 = - 26.4480                                                                    d.sub.19 = 13.8000                                                  r.sub.20 = ∞                                                                      d.sub.20 = 1.6000                                                                           n.sub.10 = 1.51633                                                                       ν.sub.10 = 64.15                        r.sub.21 = ∞                                                                      d.sub.21 = 4.4000                                                                           n.sub.11 = 1.54771                                                                       ν.sub.11 = 62.83                        r.sub.22 = ∞                                                                      d.sub.22 = 0.5000                                                   r.sub.23 = ∞                                                                      d.sub.23 = 0.6000                                                                           n.sub.12 = 1.51633                                                                       ν.sub.12 = 64.15                        r.sub.24 = ∞                                                            ______________________________________                                        f       10.311        16.425  26.211                                          D.sub.1 1.010         7.690   13.200                                          D.sub.2 0.820         1.760   0.800                                           D.sub.3 13.170        5.550   1.000                                           ______________________________________                                               D.sub.1W = 0.805, D.sub.1T = 11.971                                           |β.sub.IIT | = 0.747, |f.sub.II               |/f.sub.T = 0.319                                             ______________________________________                                    

Embodiment 4

    ______________________________________                                        f = 6.181˜34.916, F/2.06˜F/3.21                                   2ω = 50.2°˜9.4°                                     ______________________________________                                        r.sub.1 = 27.4156                                                                        d.sub.1 = 1.2000                                                                           n.sub.1 = 1.84666                                                                        ν.sub.1 = 23.78                         r.sub.2 = 17.5554                                                                        d.sub.2 = 3.7000                                                                           n.sub.2 = 1.72916                                                                        ν.sub.2 = 54.68                         r.sub.3 = 299.8897                                                                       d.sub.3 = D.sub.1                                                             (variable)                                                         r.sub.4 = -328.5711                                                                      d.sub.4 = 0.8000                                                                           n.sub.3 = 1.83400                                                                        ν.sub.3 = 37.16                         r.sub.5 = -8.7329                                                                        d.sub.5 = 1.9800                                                   r.sub.6 = -11.3687                                                                       d.sub.6 = 0.7000                                                                           n.sub.4 = 1.72342                                                                        ν.sub.4 = 38.03                         r.sub.7 = 10.0021                                                                        d.sub.7 = 2.8000                                                                           n.sub.5 = 1.84666                                                                        ν.sub.5 = 23.78                         r.sub.8 = -23.3617                                                                       d.sub.8 = D.sub.2                                                             (variable)                                                         r.sub.9 = ∞ (stop)                                                                 d.sub.9 = D.sub.3                                                             (variable)                                                         r.sub.10 = 23.3201                                                                       d.sub.10 = 2.7000                                                                          n.sub.6 = 1.65844                                                                        ν.sub.6 = 50.86                         r.sub.11 = -20.0228                                                                      d.sub.11 = 0.1500                                                  r.sub.12 = 11.2746                                                                       d.sub.12 = 2.9000                                                                          n.sub.7 = 1.58904                                                                        ν.sub.7 = 53.20                         r.sub.13 = -167.1643                                                                     d.sub.13 = 0.4600                                                  r.sub.14 = -21.8933                                                                      d.sub.14 = 6.0000                                                                          n.sub.8 = 1.80518                                                                        ν.sub.8 = 25.43                         r.sub.15 = 10.7636                                                                       d.sub.15 = 0.2200                                                  r.sub.16 = 14.0734                                                                       d.sub.16 = 2.8000                                                                          n.sub.9 = 1.58904                                                                        ν.sub.9 = 53.20                         r.sub.17 = -15.8124                                                                      d.sub.17 = D.sub.4                                                            (variable)                                                         r.sub.18 = 83.1097                                                                       d.sub.18 = 1.0000                                                                          n.sub.10 = 1.80100                                                                       ν.sub.10 =  34.97                       r.sub.19 = 5.7463                                                                        d.sub.19 = 0.3400                                                  r.sub.20 = 7.6305                                                                        d.sub.20 = 2.8000                                                                          n.sub.11 = 1.59270                                                                       ν.sub.11 = 35.29                        r.sub.21 = -22.3407                                                                      d.sub.21 = 1.0000                                                  r.sub.22 = ∞                                                                       d.sub.22 = 2.0000                                                                          n.sub.12 = 1.51633                                                                       ν.sub.12 = 64.15                        r.sub.23 = ∞                                                                       d.sub.23 = 1.0000                                                  r.sub.24 = ∞                                                                       d.sub.24 = 0.6000                                                                          n.sub.13 = 1.51633                                                                       ν.sub.13 = 64.15                        r.sub.25 = ∞                                                            ______________________________________                                        f       6.181         14.527  34.916                                          D.sub.1 1.100         12.039  18.359                                          D.sub.2 18.259        7.320   1.000                                           D.sub.3 8.307         5.967   1.509                                           D.sub.4 1.000         3.340   7.798                                           ______________________________________                                               D.sub.1W = 0.895, D.sub.1T = 17.087                                           |β.sub.IIT | = 0.679, |f.sub.II               |/f.sub.T = 0.275                                             ______________________________________                                    

wherein the reference symbols r₁, r₂, . . . represent the radii ofcurvature on the surfaces of the respective lens elements, the referencesymbols d₁, d₂, . . . designate the thicknesses of the respective lenselements and the airspaces reserved therebetween, the reference symbolsn₁, n₂, . . . designate the refractive indices of the respective lenselemnts, and the reference symbols ν₁, ν₂, . . . denote the Abbe'snumbers of the respective lens elements.

Embodiments 1 through 3 are designed as the vari-focal lens systemshaving the compositions illustrated in FIG. 1 through FIG. 3respectively.

The rear focusing method is adopted not only for using a light-weightfocusing lens unit advantageous for autofocusing and reducing the numberof the lens elements required for composing the vari-focal lens systembut also for reducing the diameter of the first lens unit. That is tosay, the rear focusing method is adopted for eliminating the necessityto move the first lens unit toward the object side for preventing theoffaxial rays from being vignetted. When the rear focusing method isselected, it is therefore desirable to adopt a composition which allowsto use a first lens unit having a small diameter.

In order to use such a first lens unit having a small diameter in a zoomlens system, it is necessary to locate the entrance pupil close to theobject side surface of the zoom lens system. For this purpose, it isnecessary for the zoom lens system to reduce the number of the lenselements arranged on the object side of the aperture stop, reducethickness of each of the lens elements and narrow the airspace requiredfor moving the movable lens units.

As an example of the conventional zoom lens system which comprises fourlens units and satisfies the above-mentioned requirement, there is knownthe zoom lens system in which the second lens unit is divided into alens unit consisting of two negative lens components and another lensunit consisting of a positive lens component, these two lens units aremoved in the same direction, while varying the airspace reservedtherebetween, for varying focal length of the zoom lens system andcorrecting the variation of image surface caused by the variation offocal length, and the compensator is omitted. That is to say, this zoomlens system comprises, in the order from the object side, a first lensunit having a positive refractive power, a second lens unit consistingof the two negative lens components, a third positive lens unit and afourth positive lens unit, and is adapted so as to perform zooming bymoving the second lens unit and the third lens unit in the samedirection while varying the airspace reserved between these lens units.

Each of the Embodiments 1 through 3 of the present invention is designedas a vari-focal lens system which has the composition similar to that ofthe zoom lens system described above. The vari-focal lens system havingsuch a composition comprises a second lens unit which has imagingmagnifications lower than 1x over the entire vari-focal range, and canbe focused, over the entire vari-focal range thereof, on an objectlocated at a short distance by moving the second lens unit toward theobject side, or is compatible with the rear focusing method. It isunnecessary for each of these embodiments to reserve a surplus airspacefor focusing since a sufficient airspace is reserved on the object sideof the second lens unit at the tele position. At the wide position ofeach of these embodiments, on the other hand, the first lens unit andthe second lens unit are close to each other, but the moving distance ofthe second lens unit for focusing is very short and a wide airspace neednot be reserved in the vari-focal lens system. Further, close-up in thevicinity of the wide position can easily be carried out by moving thethird lens unit, which is one of the lens units movable for zooming,toward the image side. Furthermore, each of vari-focal lens systemspreferred as the Embodiments 1 through 3 may be focused, only in thevicinity of the wide position, by moving the third lens unit toward theimage side.

Embodiment 4 of the present invention has the composition shown in FIG.4, wherein the vari-focal lens system comprises, in the order from theobject side, a first lens unit having a positive refractive power, asecond lens unit which has a negative refractive power and is movablefor variation of focal length of the vari-focal lens system, a thirdlens unit which has a positive refractive power and is movable forvariation of focal length of the vari-focal lens system, and a fourthlens unit.

Embodiment 4 uses the third lens unit which has imaging magnificationslower than 1x over the entire vari-focal range and is moved for zoomingin the direction opposite to the moving direction of the second lensunit. Though the second lens unit and the third lens unit are closest toeach other at the tele position, it is possible, by arranging anaperture stop between these lens units, to locate the entrance pupilclose to the object side surface of the vari-focal lens system andreduce the diameter of the first lens unit.

Embodiment 4 also uses the second lens unit which has imagingmagnifications lower than 1x over the entire vari-focal range and can befocused by moving the second lens unit toward the objectsside, or iscompatible with the rear focusing method. Further, it is unnecessary forEmbodiment 4 to reserve a surplus airspace for focusing since asufficient airspace is reserved between the first lens unit and thesecond lens unit at the tele position at which the second lens unit isto be moved for a long distance for focusing. At the wide position atwhich the first lens unit and the second lens unit are close to eachother, on the other hand, the second lens unit is moved only for a shortdistance for focusing, thereby eliminating the necessity to reserve awide airspace for focusing in Embodiment 4. Furthermore, Embodiment 4allows close-up focusing to be carried out easily by moving the thirdlens unit, which is one of the lens units movable for zooming, towardthe object side. Moreover, Embodiment 4 may be focused, only in thevicinity of the wide position, by moving the third lens unit toward theimage side.

I claim:
 1. A vari-focal lens system comprising, in the order from theobject side:a first lens unit having a positive refractive power, asecond lens unit having a negative refractive power, a third lens unithaving a positive refractive power, and a fourth lens unit, wherein saidvari-focal lens system performs variation of focal length by moving saidsecond lens unit and said third lens unit along the optical axis, andwherein said vari-focal lens system performs focusing on an objectlocated at a short distance by moving said second lens unit toward theobject side, and wherein said first lens unit does not move for focusingor zooming, said second lens unit satisfying the following condition(1):

    |β.sub.IIT |<0.95                   (1)

wherein the reference symbol β_(IIT) represents the magnification ofsaid second lens unit when said vari-focal lens system is focused on anobject located at an infinite distance at the tele position thereof. 2.A vari-focal lens system according to claim 1 wherein said second lensunit satisfies the following condition (2):

    0.15 <|f.sub.II |/f.sub.T <0.5           (2)

wherein the reference symbol f_(II) represents the focal length of thesecond lens unit and the reference symbol f_(T) designates the focallength of the vari-focal lens system as a whole at the tele position. 3.A vari-focal lens system according to claim 2 adapted to be focused atthe wide position by moving the third lens unit toward the image side.4. A vari-focal lens system according to claim 1 wherein said third lensunit has magnifications lower than 1x.